Thin films of materials such as metals, semiconductors, or metal oxide insulators are of great importance in the microelectronics industry. Fabrication of integrated circuits involves formation of high purity thin films, often with multiple layers, on patterned substrates. One of the most common methods for producing thin films is chemical vapor deposition (CVD). In thermal CVD, volatile precursors are vaporized under reduced pressure at temperatures below their thermal decomposition temperature and transported by means of a carrier gas into an evacuated chamber containing a substrate. The substrate is heated to high temperatures, and thermolysis at or adjacent to the heated substrate results in the surface deposition of the desired film. For a general reference on CVD see: Hitchman et al., eds., Chemical Vapor Deposition Principles and Applications (Academic Press, London, 1993).
Thin films have also been formed using supercritical fluids. For example, Murthy et al. (U.S. Pat. No. 4,737,384) describes a physical deposition method in which a metal or polymer is dissolved in a solvent under supercritical conditions and as the system is brought to sub-critical conditions the metal or polymer precipitates onto an exposed substrate as a thin film. Sievers et al. (U.S. Pat. No. 4,970,093) describes a standard CVD method in which organometallic CVD precursors are delivered to a conventional CVD reactor by dissolving the precursors in a supercritical fluid solvent. The solvent is expanded to produce a fine precursor aerosol which is injected into the CVD reactor under standard CVD conditions, i.e., pressures less than or equal to 1 atmosphere, to deposit a thin film on a substrate.
Louchev et al. (J. Crystal Growth, 155:276–285, 1995) describes the transport of a precursor to a heated substrate (700 K) in a supercritical fluid where it undergoes thermolysis to yield a thin metal (copper) film. Though the process takes place under high pressure, the temperature in the vicinity of the substrate is high enough that the density of the supercritical fluid approaches the density of a conventional gas. The film produced by this method had an atomic copper concentration of approximately 80% (i.e., 20% impurities). Bouquet et al. (Surf and Coat. Tech., 70:73–78, 1994) describes a method in which a metal oxide is deposited from a supercritical mixture of liquid and gas co-solvents at a temperature of at least 240° C. The thin film forms as a result of thermolysis at a substrate heated to at least 290° C.
The formation of alloys from multiple pure metal components and films containing multiple pure metal components is also of interest in microelectronic applications and device fabrication for the formation of films exhibiting, e.g., gigantic magneto resistance (GMR), increased resistance to electromigration and for modification of electrical conductivity, and for the formation of other functional layers in integrated circuits. Alloying is also used to tailor rate and selectivity for reactions over supported catalysts, improve the resistance of metal membranes to hydrogen embrittlement and to increase the hardness and corrosion resistance of barrier coatings. Mixed metal films are typically produced by physical deposition methods such as ion sputtering, which is a line-of-sight technique. In principle, CVD can also be used to produce alloy films using a combination of metal precursors. Such deposition, however, would be limited by the relative volatilities of the precursors making precise control of multi-component feed streams across the composition range difficult to achieve. Moreover, attainment of a desired composition would also depend on the relative rates of decomposition.
Thin films of palladium (Pd) and its alloys are used in technologically important applications such as catalysis, gas sensors, and H2 permselective membranes for use in gas separation and in integrated reaction/separation schemes. Moreover, Pd is a common noble metal in microelectronics, where it is used as a contact material in integrated circuits and as a seed layer for the electroless deposition of other interconnect metals. Pd films can be prepared by vacuum sputtering and electroplating. However, such techniques are generally limited to planar surfaces, limiting their applicability to applications in microelectronics where shrinking device dimensions require efficient filling of deep sub-micron, high-aspect ratio features.
High purity Pd thin films can be deposited by CVD using organopalladium compounds containing various classes of ancillary ligands as precursors. However, to maintain acceptable purity and deposition rates, temperatures usually exceed 200° C. Moreover, because CVD is often mass-transport limited, the deposited films are expected to be non-uniform, thereby limiting efficient pore-filling and/or conformal coverage of complex surfaces. Consequently, palladium CVD has not yet been commercialized.
Copper is also used in technologically important applications, including interconnect structures in microelectronic devices. Current methods of depositing copper, such as CVD and sputtering, have not been shown to provide uniform filling of very narrow (˜150 nm and less), high aspect ratio trenches or vias. As a result, copper CVD has not been practiced commercially for these applications. Other applications for copper include printed wiring boards.